1. Field of the Invention
The present invention relates to dispersion, more particularly, to dispersion of an ultraviolet absorber suitable for use in a photographic product.
2. Description of the Prior Art
Ultraviolet rays are known to cause many adverse effects on photographic products. For example, electrostatic light caused during the drying of a coated web or during transporting of the finished photographic product induces undesired sensitization. If various light sources having different intensities of UV radiation are used for shooting pictures, the color reproducibility of the photographic product may be impaired.
The possibility of a discolored image due to UV radiation is particularly great with a photographic product of the type that provides a color image of a dye other than that of metallic silver, and if the processed photographic product is put in a display case, a significant discoloration or decoloration of the image may occur owing to sunlight containing a great amount of UV radiation.
If a photographic support made of polyethylene-coated paper is used, ultraviolet radiation deteriorates polyethylene and this may lead to cracking in the support.
A photographic product contains various organic additives and they may often stain the non-image area since they form a colored material when they are decomposed under light. For example, the white background of color photographic paper is subject to yellowing upon illumination with light, and a method to eliminate this problem of light stain is desired.
In order to mitigate the deleterious effects of ultra-violet radiation, a UV absorber is generally incorporated in one or more of the silver halide emulsion layers or non-sensitive photographic layers of the photographic product. While a great number of compounds have been proposed for use as the UV absorber, the latter is required to have the following characteristics:
(1) the absence of absorption in the visible range;
(2) the absence of adverse effects on the photographic product;
(3) high solubility in a high-boiling organic solvent; and
(4) a sufficient fastness to light to minimize the effect of prolonged illumination by light.
Typical compounds for use as the UV absorber are shown in U.S. Pat. Nos. 2,685,512, 2,719,086, 2,739,888, 2,739,971, 2,747,996, 2,784,087, 2,811,461, 3,112,338, 3,168,492, 3,206,431, 3,253,921, 3,692,525, and 3,754,919; Japanese patent publication Nos. 4,786/1967, 26,139/1974, 25,337/1975, 12,587/1980 and 36,984/1980; and International Publication No. 01473/1981 and European patent publication No. 57,160.
Several techniques have been proposed for dispersing the ultraviolet absorber. According to the first method, which is generally referred to as the Fischer type dispersion method, the absorber is dispersed as finely divided particles. According to the second method, a UV absorber and other necessary components are dissolved in a water-miscible organic solvent, and then a fillable polymer latex and water sufficient to render the UV absorber and other necessary components insoluble are gradually added to the solution, thereby incorporating the absorber in each particle of the polymer latex. For details of the miscible organic solvent and fillable polymer latex, see Japanese patent application (OPI) Nos. 59,942/1976 and 59,943/1976 (the symbol OPI as used herein means an unexamined published Japanese patent application).
In the third method, a UV absorber and other necessary components are dissolved in a substantially water-insoluble high-boiling organic solvent, and the solution is finely dispersed in a hydrophilic protective colloid to obtain an emulsion of O/W type. Details of this method are given in U.S. Pat. Nos. 2,322,027, 2,801,170, 2,801,171, 2,870,012 and 2,991,177. A specific example of applying this method to a UV absorber for use in photographic materials is shown in U.S. Pat. No. 2,739,888.
Of these three methods, the last mentioned method is most extensively used today because it is adapted to operation on an industrial scale (it causes no increase in the viscosity of the coating solution containing the UV absorber) and provides a sharp peak of absorption by the UV absorber. However, even this method has the following two problems that must be solved before it can be put into actual operation. One problem is well known and relates to the low stability of the emulsion product. Because of this low stability, the dispersed particles become coarse or come out of dispersion in the form of crystals, so as to induce various troubles in the manufacturing process. The other problem has been identified first by the present inventors and it is the low stability in a dark place of a photographic coating containing the UV absorber. This tendency is particularly great under hot and humid conditions, and the coating soon loses its ability to absorb ultraviolet rays under such conditions. This is indeed a serious problem since color photographic papers must sometimes be displayed during a rainy season, or the place where they are displayed may present an environment of locally high humidity as in a semi-closed display case or substantially closed picture-frame or photo-stand.
Several methods have been proposed for solving the first problem by stabilizing the dispersion of a UV absorber, and they can be divided into two groups. The first group of stabilization methods depends on improving the solubility of the UV absorber in a high-boiling organic solvent by introducing a change in its chemical structure and is shown in Japanese patent publication Nos. 26,187/1967, 12,587/1980 and 36,984/1980; and International Publication No. 01473/1981 and European patent publication No. 57,160. However, some of the compounds shown are liquid at room temperature and hence are difficult to handle and cause a loss in the yield of the final product. This unavoidably results in a high cost of manufacture. Furthermore, these compounds are difficult to purify and cause variations in the composition of the final product from one batch to another.
The second group of methods is characterized by using two or more ultraviolet absorbers and is shown in Japanese patent publication Nos. 5,496/1973, 30,493/1973 and 41,572/1973; and Japanese patent application (OPI) No. 85,425/1978.
These methods are effective to some extent in improving the stability of the dispersed particles of a UV absorber against crystallization. However, they are far from being completely satisfactory in that they have a limited effectiveness in preventing the coarsening of dispersed particles over the course of time. Furthermore, none of these methods are capable of solving the second problem, i.e., low storage stability in coating, especially under hot and humid conditions.
Accordingly, it is desired to develop a dispersion of ultraviolet absorber that is suitable for use in the preparation of a highly stable photographic coat and which retains a sufficiently stable dispersibility to provide improved stability in the photographic coating even under hot and humid conditions.